Processes for the manufacture of acetic acid from methanol by carbonylation are operated extensively throughout the world. A thorough review of these commercial processes and other approaches to accomplishing the formation of acetyls from single carbon sources is described by Howard et al. in Catalysis Today, 18 (1993) 325-354. All commercial processes for the preparation of acetic acid by the carbonylation of methanol presently are performed in the liquid phase using homogeneous catalyst systems comprising a Group VIII metal and iodine or an iodine-containing compound such as hydrogen iodide and/or methyl iodide. Rhodium is the most common Group VIII metal and methyl iodide is the most common promoter. These reactions are conducted in the presence of water to prevent precipitation of the catalyst. The catalyst precipitation is a serious problem during the flashing process whereby the liquid products exiting the carbonylation reactor are subjected to a partial release in pressure. The pressure release causes vaporization of a portion of the products and a decrease in the temperature resulting from the heat of vaporization. If too much water is removed during the flashing process, precipitation of the rhodium catalyst will occur. U.S. Pat. No. 5,144,068 describes the inclusion of lithium in the catalyst system which allows the use of less water in the Rh-I homogeneous process.
The precipitation of the catalyst is reduced through the use of quaternary phosphonium or quaternary ammonium iodides as soluble components in liquid phase carbonylation processes for catalyst promotion and catalyst stabilization in catalyst purification processes. U.S. Pat. No. 4,430,273 describes a process for making acetic anhydride from the carbonylation of methyl acetate in the presence of a Group VIII metal in a carboxylic acid solvent containing a heterocyclic quaternary nitrogen compound. U.S. Pat. No. 4,333,884 describes a process for making acetic anhydride from the carbonylation of methyl acetate in the presence of a Group VIII metal in a carboxylic acid solvent containing a heterocyclic quaternary nitrogen or phosphorous compound and a zirconium compound. The processes described in U.S. Pat. Nos. 4,430,273 and 4,333,884 require a carboxylic acid solvent component and are operated in a mode where the product is removed from the reaction zone as a liquid.
The troublesome flash process can be avoided by conducting the carbonylation reaction in the vapor-phase using heterogeneous catalysts. Schultz, in U.S. Pat. No. 3,689,533, describes the use of supported rhodium as a heterogeneous catalyst for the carbonylation of alcohols to carboxylic acids in the vapor phase in the presence of a halide promoter. In U.S. Pat. No. 3,717,670, Schultz describes a similar supported rhodium catalyst in combination with promoters selected from Groups IB, IIIB, IVB, VB, VIB, VIII, (current notations: 11, 3, 4, 5, 6 and 8-10 respectively) lanthamide and actinide elements of the Periodic Table. Uhm, in U.S. Pat. No. 5,488,143, describes the use of alkali, alkaline earth or transition metals as promoters for supported rhodium for the halide-promoted, vapor phase methanol carbonylation reaction. A serious problem associated with vapor phase carbonylation reactions using solid phase catalysts is the removal of the heat of reaction. Since the thermal conductivity of vapor is much less than that of the corresponding liquid, the reactor used in a vapor phase process must have a high surface area to facilitate the removal of the heat of reaction. The complexity and cost of these high surface area reactors negates much of the potential beneficial aspects of vapor phase carbonylation processes.
Ionic liquids have been used as solvents for synthetic reactions and transistion metal-catalyzed processes including carbonylation reactions. A review of ionic liquids is provided by Welton in Chemical Reviews 99 (1999) 2071-2083. Knifton et al. in U.S. Pat. No. 4,366,259 describe a process for the preparation of acetic and propionic acid by homologation of carbon monoxide in the presence of hydrogen and a ruthenium-cobalt catalyst system dispersed in a low-melting phosphonium or ammonium salt. The selectivity of this process toward either acetic or propionic acid was low. In addition, this process was conducted under batch conditions and required extended reaction times, high temperatures, and high pressures. Such forcing conditions would make this carbonylation process difficult and uneconomical to operate. Tanaka et al., in Green Chemistry 3 (2001) 76-79, describe the palladium-catalyzed carbonylation of aryl halides with alcohols in the ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate or hexafluorophosphate. The process of Tanaka et al. does not avoid many of the catalyst handling and processing disadvantages encountered in traditional liquid phase carbonylation processes.
The liquid phase carbonylation processes discussed above require separation of the volatile reaction products and starting materials from the less volatile catalyst components by distillation from the reaction vessel or by flashing of the reaction solution at reduced pressures. These separation processes are frequently complicated, require expensive corrosion resistant equipment, involve extensive recycle and processing of catalyst residues, and often result in loss of catalyst values. Heterogeneous processes avoid the flash process but cannot remove heat at a sufficient rate to attain high production rates. Thus, there is need for a carbonylation process which provides for simple product separation while maintaining a stable catalyst environment and gives high reaction rates with efficient heat removal from the reaction zone.